What was found
AT CRUISE FLT OF 2000' MSL & 120 KNOTS, 4 MINUTES INTO FLT, AN ODOR OF "BURNING WOOD" DETECTED IN CABIN AREA & WITHIN SECONDS, IN COCKPIT. HAD JUST FLOWN 2 MILES OF A FARMER BURNING BRUSH ON THE GROUND AND THE SMOKE PLUME WAS ABOUT 1500' TALL. CREW DISCUSSED POSSIBILITY OF THE RESIDUAL SMOKE FROM GROUND FIRE ENTERING CABIN/COCKPIT AREA. WIND DIRECTION & STRENGTH OF ODOR DID NOT MAKE SENSE. AFTER 20 SECONDS, FLT NURSE REPORTED THAT SHE HAD JUST SEEN A WISP OF SMOKE EMITTING FROM OVERHEAD AIR VENT. ODOR & INTRUDING SMOKE COULD NOT HAVE BEEN FROM GROUND FIRE. TURNED OFF AIR VENT, OPENED COCKPIT SLIDING WINDOW TO GET RID OF ODOR & SMOKE & ENTERED A DESCENDING LT BANK TO AIRPORT. LANDED WITH NO OTHER INCIDENT & PERFORMED AN EMERGENCY SHUTDOWN & EGRESSED ACFT. OPENED COWLINGS ON ACFT TO EXAMINE ENGINE & OTHER COMPONENTS FOR SIGNS OF BURNING, OR LINE CHAFING, ETC. & FOUND NONE. A STRONG ODOR STILL EMITTED FROM OVERHEAD VENT IN CABIN. DIAGNOSED PROBLEM TO BE OVERHEAD FAN BLOWER OVERHEATED & "FREEZE UP". ACFT RETURNED TO BASE WITHOUT ANY FURTHER INCIDENTS & RETURNED TO SERVICE.